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Chemistry > Acidic Environment 4 > Flashcards

Acidic Environment 4 Flashcards

1
Q

Lavoisier

A

led to believe that all acids contain oxygen and this is what gave rise to their acidity.(soon proven false as many oxygen containing compounds are basic and some acids (e.g. hcl) don’t contain oxygen.

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2
Q

Davy

A

Showed that HCl doesn’t contain oxygen. Metals could displace H from acids. Proposed that acids were substances that contained H.

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3
Q

Arrhenius

A

Proposed that acids were neutral substances that in solution produced a positive H ion and a negative ion. Bases ionise to produce OH ions in solution and realised strength of acid depended on its degree of ionisation. Able to explain that neutralisation was reaction between H ion of an acid and the OH ion of a base.

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4
Q

Limitations of Arrhenius idea

A
  • only applied to aq solutions
  • Didn’t explain that the ionisation of an acid is a reaction between the acid molecule and the solvent.
  • only considers substances that contain H
  • Cant explain why some salts can act as acids
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5
Q

Bronsted-Lowry theory

A
  • acids are proton donors
  • bases are proton acceptors
  • if in a particular solvent, a substance HA has a greater tendency to give up protons than the solvent then HA is an acid.
  • HA (aq) + H20 (l) H3O+ (aq) + A- (aq)
    HA giving up proton (therefore is the acid) water is accepting the proton (therefore is a base)
    Shows that acidity doesn’t just depend on the structure of a substance it also depends on the properties relative to those of the solvent/reactant.
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6
Q

Conjugates

A

Every acid has its conjugate base and every base has its conjugate acid.
HA (aq) + H20 (l) H30+ (aq) + A- (aq)
acid conjugate base

Strong acid has weak conjugate base
Weak acid has a strong conjugate base.
strong base has weak conjugate acid
weak base has a strong conjugate acid

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7
Q

When is a salt formed

A

When an acid is neutralised by a base.
Can be acidic, basic or neutral.
Hydrolysis reactions between weak acids and bases.
Many anions and cations can act as acids and bases

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8
Q

Basic salt

A

E.g sodium acetate
Contains acetate ion which is the CB of the WA (acetic acid)
CH3COO- + H2O CH3COOH + OH-
Acetate ion accepts a proton from water to form the largely undissociated acetic acid and a OH ion.
In solution, sodium acetate has a pH >7.
WA + SB –> basic solutioin (pH >7) in water

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9
Q

Acidic salt

A

E.g Ammonium chloride.
Contains the ammonium ion that is the CA of a WB (ammonia)
NH4+ + H2O NH3 + H3O+
Ammonium ion donates proton to water to form NH3 and the hydronium ion which makes the solution acidic.
SA + WB –> acidic solution (pH

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10
Q

Neutral salt

A

E.g. Sodium Chloride
SA (HCl) and the SB (sodium hydroxide).
Chloride ions or NA ions don’t show any tendency to react with water.
SA + SB –> neutral solution (pH=7) in water
WA + WB –> neutral solution (pH=7) in water.

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11
Q

Amphiprotic substances

A

Some ions have the ability to either donate or accept protons.
e.g. WATER
NH3 + H2O –> NH4+ + OH- (Water donates proton)
HSO4- + H2O –> SO42- + H3O+ (water accepts proton)
* molecules or ions that can behave as both acids and bases in different circumstances.
e.g.(HSO4-, HPO4-) must contain at least one H if it is to donate a proton.

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12
Q

Amphiprotic substance ( hydrogen carbonate ion)

A

in presence of an acidic solution it acts as a base:
HCO3- + H3O+ –> H2CO3 + H2O
In presence of a SB it acts as an acid:
HCO3- + OH- –> CO32- +H2O
since H2CO3 isn’t very soluble in water, it breaks up to form water and carbon dioxide:
H2CO3 –> H2O + CO2
This is why when an acid is added to Hydrogen carbonate ions, you will see a gas evolving.

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13
Q

Neutralisation as a proton transfer reaction which is exothermic

A

acid + base –> salt + water
e.g. HCL + NaOH –> NaCl + H2O
(HCl acid transfers proton to the base OH- to form water and Na+ is a spectator ion)
net ionic reaction is :
H+ (aq) + OH- (aq) –> H20 (l)
EXOTHERMIC: as acid is added to a basic solution, the temp rises. Enthalpy change for neutralisation reactions is -57 kJ/Mol

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14
Q

Titrations

A
  • neutralisation reactions determine conc. of unknown samples. ( volumetric analysis)
  • standard solution ( of known conc) is added to another of unknown until reaction is complete.
  • point where reaction is complete is called equivalence point. Sudden change as solution changes from acidic to alkaline etc.
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15
Q

steps in titration

A
  1. rinse a clean burette with small amount of solution of known concentration.
  2. Fill burette with known solution until zero mark. (TITRANT)
  3. Use pipette to measure a given volume of the unknown conc. solution into a flask and place under burette.
  4. add indicator to flask
  5. slowly run titrant into the flask swirling continuously. (add until just changes colour.)
  6. read volume of titrant delivered on the burette
  7. repeat at least 3 times
  8. average 3 closest titrant volumes and calculate conc of solution of unknown concentration.
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16
Q

End points of titration

A
  • SA/SB pH =7 –> Bromothymol blue (6-7.6)
  • WA/WB doesn’t yield reliable result with indicators as there is no sudden change in pH around equivalence point.
  • WA/SB ph = 9 –> phenolphthalein (8.2-10)
  • SA/WB pH=4 –> methyl orange (3.1-4.4)
17
Q

Standard solution

A

solution of accurately known concentration.
Made by dissolving a substance called a primary standard.
Needs to be a substance of sufficiently high purity and stability.
E.g. sodium carbonate.
Can be accurately weighed, dissolved in water and then made up to desired bolume.

18
Q

What is a buffer

A

solution that tends to resist any great change in pH when small quantities of acid or base are added to it. It consists of:
* WA and the salt of that WA
OR
* WB and the salt of that WB

19
Q

Buffers in biological systems

A
  • enzymes (biological catalysts) work best at very narrow pH ranges so any slight changes in pH can upset the chemistry of living things.
    Haemoglobin is complex iron containing molecule found in bloodstreams and combines with oxygen to transport it around our blood systes to the cells needed.
    Haemoglobin + oxygen –> oxyhaemoglobin.
    Haemoglobin (Hb) and oxyhaemoglobin are both basic and in solution they form their conjugate acids.
    HbH+ (aq) + O2 (aq) HbO2 (aq) + H+ (aq)
  • pH of the bloodstream is kept @ 7.4.
  • When pH
20
Q

assess the use of neutralisation reactions as a safety measure or to minimise damage in accidents and chemical spills.

A

Exothermic (ethalphy change : -57kJ/Mol)

  • Add acid to water. (minimises heat generated as water can absorb large amount of heat for each degree rise in temp)
  • spills of acids or bases can be neutralised using WB. or WA
  • Acids wastes from factories cant be discharged into waterways until neutralised and diluted. Settling tank is useful to allow heat from neutralisation reactions to dissipate before wastes are disposed.
  • wear safety gear when handling acids/bases.

Chemistry (9 decks)

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